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Electronics => Repair => Topic started by: SL4P on May 10, 2015, 11:19:42 am

Title: Laser diode & LED - what's going on here?
Post by: SL4P on May 10, 2015, 11:19:42 am
I've been asked to repair a handheld laser diode 'probe' with a status LED included.
The original had been mangled - and basically all old connections were physically ripped apart (under epoxy & hot glue!)

The attached drawing is what I 'think' the original wiring was set up as, but it doesn't work.
Interestingly - the unit's controller also shows a strange symptom...

If the pushbutton is pressed (to start the timed cycle), and there is no LED or laser diode in circuit - the controller displays 000...  If the diodes are in circuit (paralleled across the 2V line) the controller shows  a different value*... so I guess it's displaying some error or status  measuring the current draw ??

Any ideas what the circuit is likely to have been?
It was a real dog's breakfast - apart from being totally trashed internally!

(* the values displayed are even more confusing -- I have four completely separate controllers, and three of them show different numeric values when the LEDs are in circuit!)

-- and sadly, no manual or schematics of any sort.  Just an (old) micro on each controller board.
Title: Re: Laser diode & LED - what's going on here?
Post by: Kleinstein on May 10, 2015, 03:11:00 pm
A white LED needs something like 3 V to work. A normal NIR laser (e.g. 780 nm) will need at least 1.6 V (more likely 1.8-2 V). So 2 V will not work. Also 3 V at the 35 ohms resistor would mean something like 100 mA for the laser diode before the LED turns on  - that's quite a lot.

It is hard to say how the parts should be connected. An alternative circuit would be the 35 Ohms in series with the laser diode and the whole thing parallel to the white LED.

What wavelength and power is the laser supposed to be ?
Title: Re: Laser diode & LED - what's going on here?
Post by: SL4P on May 10, 2015, 04:27:20 pm
Zero documentation but apparently the laser is 50mw @ 830nm
I'll have it specked when it 'works'.

BTW with the laser and LED in parallel straight across the 2V drive - both are illuminating - just that odd controller display issue!!?

Sadly the controllers are soldered to rear facing connectors which are pre-mounted through the front panel - sigh.